catalytic growth of cnt and nano fibers on vermiculite

8/2/2019 Catalytic Growth of CNT and Nano Fibers on Vermiculite

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Catalytic growth of carbon nanotubes and nanofibers on vermiculiteto produce floatable hydrophobic nanosponges for oil spill remediation

Flavia C.C. Moura a,*, Rochel M. Lago b

a Departamento de Qu mica ICEB, Universidade Federal de Ouro Preto UFOP, Ouro Preto, MG, 35400-000, Brazilb Departamento de Qu mica ICEx, Universidade Federal de Minas Gerais UFMG, Belo Horizonte, MG, 31270-901, Brazil

1. Introduction

Catalytic chemical vapour deposition (CVD) synthesis of carbon

nanotubes (CNT) using a large variety of catalysts, e.g. Fe/Mo, Ni,

Co, and different carbon sources, e.g. CH4, C2 and C3 hydrocarbons,

CO, ethanol, has been extensively investigated in the last years

[1,2]. Due to its relative simplicity, low cost, high yields and the

possibility to control several features of the CNT the CVD process

has proven to be the most promising large scale carbon nanotubes

preparation process. Differentoxides suchas Al2O3, SiO2, MgO have

been used as support to grow CNT [36]. Well defined surfaces,

such as silicon wafer have also been used to grow highly organized

aligned CNT known as carpets [7,8].

In this work, we have used the surface of vermiculite to grow

CNT and nanofibers via CH4CVD synthesis. Vermiculite, a clay

mineral, is a very interesting layered material with many potential

industrial and environmental applications [9,10]. Upon sudden

heating at temperatures higher than 700 8C inter lamellae watermolecules evaporate abruptly, separating packets of layers that

produce a highly developed porous structure. This expanded

vermiculite (EV) shows a volume up to 20 times greater and floats

on water due to the decrease in density, ca. 0.050.30 g cm3

[9,11]. This vermiculite has been used to removeoil spilled onwater

due to the strong capillary action of the slit shapedpores [1216]. A

strong drawback of vermiculite is the high water uptake and low

absorption of the hydrophobic organic contaminants due to the

strong hydrophilic clay surface. Several works and patents report

different processes to hydrophobize the vermiculite surface using,

for example, siloxanes [17] and polymer coating [13]. However, all

these processes were relatively complex and produced relatively

low oil removal capacity.

Hereon, we report the CH4 CVD on the vermiculite surface to

produce a dense CNT and nanofibers complex entangled structure.

This nanostructured carbonproducesa highlyhydrophobic material

with a spongeeffectconferring to theabsorbent a high oil removal

capacity. This is one of the first environmental applications of

carbon nanotubes.

2. Experimental

The vermiculite used in this work has the approximate

composition (Al0.30Ti0.04Fe0.63Mg2.00)(Si3.21Al0.79)O10(OH)2Mg0.13-Na0.02K0.10(H2O)n. The exfoliation was carried out by introducing

the vermiculite in a quartz tube at 1000 8C for 60 s. The exfoliated

vermiculite was impregnated with solutions of Fe(NO3)3 and

MoO2(acac)2 using H2O or methanol as solvents at different

concentrations. The impregnated EV was dried at 80 8C for 3 h and

submitted to the CVD process. The different prepared EV (ca. 1.0 g)

was placed in a quartz tube of 40 mm diameter and heated at

10 8C min1 to the reaction temperature of 900 8C under H2/Ar flow

(150 mL min1) and the temperature was kept at 900 8C for 1 h to

pre-reduce the catalyst precursors. The material was then

immediately submitted to a CVD with CH4/Ar (1/1 mixture at

1 atm and 600 cm3 min1) at 900 8C for 1 h. The reaction lasted for

Applied Catalysis B: Environmental 90 (2009) 436440

A R T I C L E I N F O

Article history:

Received 30 September 2008Received in revised form 19 March 2009

Accepted 2 April 2009

Available online 10 April 2009

Keywords:

Carbon nanotubes

Vermiculite

Adsorbents

Environmental application

A B S T R A C T

In this work, chemical vapour deposition (CVD) synthesis of carbon nanotubes (CNT) and nanofibers on

the surface of expanded vermiculite (EV) was used to produce a highly hydrophobic floatable absorventto removeoil spilledon water. XRD, SEM, TG andRaman spectroscopy showed thatthe carbon nanotubes

and nanofibers grow on FeMo catalyst impregnated on the EV surface to form a sponge structure. As a

result of these carbonaceous nanosponges the absorption of different oils remarkably increased ca. 600%

with a concomitant strong decrease of the undesirable water absorption.

2009 Published by Elsevier B.V.

* Corresponding author. Tel.: +55 31 3559 1710; fax: +55 31 3559 1707.

E-mail address: [email protected] (Flavia C.C. Moura).

Contents lists available at ScienceDirect

Applied Catalysis B: Environmental

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / a p c a t b

0926-3373/$ see front matter 2009 Published by Elsevier B.V.

doi:10.1016/j.apcatb.2009.04.003
mailto:[email protected]://www.sciencedirect.com/science/journal/09263373http://dx.doi.org/10.1016/j.apcatb.2009.04.003http://dx.doi.org/10.1016/j.apcatb.2009.04.003http://www.sciencedirect.com/science/journal/09263373mailto:[email protected]


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10 min, unless otherwise stated. After the reaction, the systemwas

cooled under Ar flow.

The vermiculitecarbon composites were characterized by

Raman spectroscopy (Renishaw) withthe excitation wavelength of

633 nm and a laser spot size of 20mm with confocal imaging

microscope. The spectra taken at ten different points were

averaged to minimize dispersion of the sample position.

The powder XRD data were obtained in a Rigaku model

Geigerflex using Cu Ka

radiation scanning from 28 to 758 at a scan

rate of 48 min1. Scanning electron microscopy (SEM) analyses

were carried out in a Jeol JKA 8900RL. TG analyses were carried out

in a Hi-Res TGA 2950 thermogravimetric analyser (TA) instrument,

with a constant heating rate of 10 8C min1 under air flow

(100 mL min1). BET surface area was carried out in an Autosorb

1 Quantachome using 22 cycles N2 adsorption/desorption and the

mercury porosimetry measurements were performed in a Quan-

tachrome type PoreMaster 60 porosimeter.

The absorption experiments were carried simulating an oil

spilling situation using 10 mL of contaminant, i.e. soy bean cooking,

mineral oil and diesel, in 100 mL water. The produced hydro-

phobized vermiculite (100 mg) was then added to the suspension it

of. After 5 min the vermiculite was removed using a simple metal

sieve and left still for 3 min to drain the excess oil and water.

Previous optimizationexperimentsshowed thatthe EV immediatelysaturates with both oil and water and contact times of 5 min are

enough to reach equilibrium. Also,the draintimein the sieve has no

significant influence on the amount of oil and water absorption.

After absorption, the materials were weighed to determine the total

amount of oil and water retained. For vegetable and engine oils the

materials were dried at 80 8C overnight to remove water and

weighed again to determine both the water and oil absorptions. For

the experiments with diesel, the sample cannot be dried at 80 8C

since a significant part of diesel will volatilize at this temperature.

Therefore after absorption experiment the sample was weighed to

determine the total amount of water and diesel absorbed. The

sample was then extracted with 20 mL hexane and analyzed by gas

chromatography (Shimadzu 17A equipped with FID capillary

column Alltech EC-Wax 30m). From a simple calibration, theamount of diesel absorbed can be calculated from the total area of

the peaks.

3. Results and discussion

3.1. Preparation and characterization of the composite EV/

nanostructured carbon

The composites vermiculite/carbon nanostructures were pre-

pared by impregnation of Fe(NO3)3 and Mo(acac)2O2 in different

concentrations on the vermiculite surface from methanol or water

solutions. Table 1 shows the prepared precursors with different Fe-

Mo/EV contents and the solvent used in the impregnation.

The precursors were pre-reduced under H2/Ar flow at 900 8C

for 1 h and immediatelysubmitted to a CVD withCH4/Ar at900 8C

also for 1 h.

The obtained materials were characterized by SEM, TG, Raman

spectroscopy and XRD. After CVD all samples were completely

black due to carbon deposition. The samples prepared using water

as solvent in the impregnation step were very brittle and fragile

with almost total collapse of the lamellar vermiculite structure. As

a resultof this collapsethe Fe(W) samples showed a small decrease

on the surface area (Table 2). On the other hand, samples prepared

with methanol showed the lamellar and porous structure intactand mechanically resistant.

Fig. 1a shows the scanning electron microscopy of the

exfoliated vermiculite before CVD with regular flat surface

lamellae and a slit-type porous structure. After CVD, the sample

Fe1(W) appears very brittle and no filaments or deposited material

could be observed (Fig. 1b and c). On the other hand, SEM images

for the Fe2(M) and Fe2Mo0.30(M) (Fig. 2di) suggests that the EV

completely changes the texture. A further magnificationof the SEM

image clearly showed a surface completely covered with carbon

filaments with nanometric diameters and several micrometers

long (Fig. 1ei).

These materials with different forms of carbon were character-

izedby Ramanspectroscopy.Spectra obtained with thelaser633 nm

(Fig. 2) showed the presence of broad G bands at ca. 1595 cm1

relatedto graphiticlayersand thedisorder D peakat 1330 cm1.The

low IG/ID ratio of the samples Fe(w) without Mo (0.61.0) suggests

the presence of high quantity of amorphous carbon or defects in the

carbon structure. On the other hand, when Mo is introduced in the

sample, i.e. Fe1Mo0.15(M) and Fe2Mo0.30(M), a very high IG/ID ratio of

3.2 and 7.9, respectively, was observed, suggesting the formation of

well graphitized carbon. Also, it is interesting to observe that in the

presence of Mo, the Raman spectra show strong bands at low

wavenumber at 133248 cm1 related to the formation of single

wallcarbonnanotubes (SWCNT) withcalculateddiameters between

0.9 nm and 1.8 nm [18].

TG analyses for all Fe(M)/EV and FeMo(M)/EV samples after

CVD showed a weight gain from 150 8C up to 550 8C (Fig. 3). This

Table 2

General properties of the EV composites prepared.

Sample BET surface

area (m2 g1)

Hg porosimetry

(cm3 g1)

Ca (%) Forms of carbonb Average oilabsorption (g g1) Waterabsorption (g g1)EV 5 1.9 More amorphous 0.5 3.7

Fe1(W) 3 Ca. 0.1 More amorphous

Fe2(W) 3 Ca.0.1 More amorphous

Fe3(W) 2 1.6 Ca.0.1 More amorphous

Fe1(M) 7 Ca.0.7 More amorphous 1.7 0.5

Fe2(M) 9 1.8 ca.2 More amorphous 1.3 0.5

Fe1Mo0.15(M) 15 1.8 ca.2 SWCNT and graphitic carbon 3.2 0.5

Fe2Mo0.30(M) 18 1.7 ca.2 SWCNT and graphitic carbon 2.0 0.5

a From TG.b

Obtained from the IG/ID ratio and from SEM analyses.

Table 1

Conditions used to prepare the different vermiculite precursors for the

CVD synthesis.

Sample Fe (wt%) Mo (wt%) Solvent

EV

Fe1(W) 1 H2O

Fe2(W) 2 H2O

Fe3(W) 3 H2O

Fe1(M) 1 MeOH

Fe2(M) 2 MeOHFe1Mo0.15(M) 1 0.15 MeOH

Fe2Mo0.30(M) 2 0.3 MeOH

F.C.C. Moura, R.M. Lago/ Applied Catalysis B: Environmental 90 (2009) 436440 437
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increase is related to the oxidationof iron, molybdenium andother

metals in the EV which were reduced by H2 during the CVD

process. Weight losses observed at higher temperatures should be

related to carbon oxidation and were used to estimate the amount

of carbon deposited. TG of samples prepared by Fe impregnation

with water, showed in general a weight loss of only ca. 0.10.2%,

suggesting that no significant amount of carbon material was

formed. This result suggests that water is not a good solvent for Fe

impregnation on EV. Simple soaking in water strongly decreasesthe EV mechanical resistance. Moreover, as vermiculite is a cation

exchange material, the Fe3+ from solution can migrate into the EV

lamellar structure. Upon pre-reduction with H2, the interlayer Fe3+

is converted to Fe0 which might lead to a collapse of the local clay

structure. Also, during impregnation, Fe3+ in H2O can hydrolyze to

produce insoluble large particles of Fe(OH)3 which should be less

active to produce carbon during CH4 CVD.

On the other hand, the samples prepared with methanol

Fe1(M) and Fe2(M) showed weight losses of ca. 0.7% and 2%,

respectively, suggesting that the Fe content directly affects the

amount of carbon deposited. The relatively high oxidation

temperature, i.e. 520 8C, indicates thepresence of a well organized

carbon. Inthe catalysts FeMo(M), ca.2% carbon was formed witha

lower oxidation temperature (440 8C), suggesting that Mo has animportant effect on the types of carbon deposited.

It was observed by XRD (Fig. 4) that the CVD process does not

affectthe EVcrystalline. Itcan be observed a small peak at 468 after

the reduction step suggesting the presence Fe metal. It was not

possible to observe any reflection peaks related to Mo.

Fig. 3. TG of the composite EV/nanostructured carbon material, in air.

Fig. 1. SEM images of EV before hydrophobization (a) and after CVD with CH4 at 900 8C for 1 h for Fe1(W) (b and c), Fe2Mo0.30(M) (dg) and Fe2(M) (h and i).

Fig. 2. Raman spectra of the composite EV/nanostructured carbon material.

F.C.C. Moura, R.M. Lago/ Applied Catalysis B: Environmental 90 (2009) 436440438


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3.2. Oil removal studies

For the oil removal studies three different model contaminants

were used, i.e. vegetable oil (soy bean), mineral oil and diesel. The

obtained absorption capacities for water and for the different oils

are shown in Fig. 5

It can be observed that the EV before CVD absorbs a high

amount of water ca. 3.5 g g1 but low absorption capacities for the

oil contaminants, ca. 0.5 g g1. For the composites Fe1(M) and

Fe2(M) better results were obtained with the decrease of the H2Oabsorption to 0.5 g g1 and a general increase on the oil removal

capacity 1.31.7 g g1. On the other hand, for the composite

Fe1Mo0.15(M) a remarkable improvement is observed with H2O

absorption strongly decreasing to 0.5 g g1 with oil removing

capacities of 3.5 g g1, 3.7 g g1and 3.8 g g1 for vegetable, engine

and diesel, respectively.

In order to rationalize the changes observed on the absorption

properties, Table 2 compares several parameters, such as the BET

surface area, Hg porosimetry results, carbon content and its forms

for the different composites.

BET surface areas do not vary significantly after CVD for the

samples Fe(W) and only a small increase is observed for the Fe(M).

On the other hand, an important increase on the surface area is

observed for the samples FeMo(M) reaching 18 m

2

g

1

. This

increase is likely related to thelarge area on the walls of nanotubes

and nanofilaments grown on the EV surface. As the amount of

carbon deposits is small, ca. 2 wt%, no significant change was

observed in the macropore volume, as suggested by Hg porosi-

metry. When these results are analyzed three main factors seem

important for the absorption properties of EV, i.e. the hydro-

phobicity of the surface, the nanotubes/nanofibers sponge

structure and the pore volume available for absorption. All

composites were covered with a layer composed of different

forms of carbon, e.g. amorphous, graphitic, fibers, and tubes. All

these carbon forms are very hydrophobic in nature and should

produce a very water repellent environment. For this reason the

water absorption by EV decreases from 3.5 g g1 to ca. 0.5 g g1

after CVD. Also, this hydrophobic character leads to a better

interaction with the hydrophobic oil molecules and should lead to

a general increase in oil absorption. Nevertheless, the composites

FeMo(M) showed an unexpectedly high oil absorption. Although

the reasons forthis behavior arenot clear they can be related to the

carbon nanostructures present in these composites. The nanotubes

and nanofibers on theEV lamellae shouldofferextra surface for the

interaction with the oil contaminants as observed by BET. Also,

these entangled tubes and fibers produce a sponge-like structure

which should have a strong capillary effect favoring the up take of

the more viscous oils. In general, oils have much higher viscosity,e.g. 7080 cP for vegetable, diesel and crude oils, compared to

water (1 cP) and the penetration of EV pore structure should be a

critical step.

As expanded vermiculite absorbs liquids into the inter-lamellar

space, the third important factor affecting the oil absorption is the

pore space available. It can be considered that the carbon

structures produced in the inter-lamellae space do not cause a

significant change in pore volume as suggested by Hg porosimetry.

For this reason the maximum amount of liquid absorbed

(water + oil) is nearly the same for EV and FeMo(M), considering

that water and oil have similar densities, 1 and 0.90.95 g cm3,

respectively. Moreover, materials withhigher BET surface area, e.g.

FeMo(M) do not show increased liquid (oil + water) removal. This

reinforces the idea that the pore volume filling by absorptionrather than adsorption is more important to define the amount of

retained liquid.

Fig. 5 also shows the oil and water absorption for two other

modified EV absorbents described in the literature [13] and

commercially available [17]. Although different EV were used to

produce these materials which should have some effect on the oil

removal capacity, it can be observed that the composite Fe1Mo0.15is much more efficient with higher oil removal capacity and low

water retention. This much lower water absorption reflects the

strong hydrophobic character of the carbon deposits compared to

the other hydrophobization processes used in refs. [13] and [17].

We have also carried an experiment with engine oil spilled in

waterwith3.5% dissolvedNaCl to simulate seawater. The obtained

results showed no significant effect of NaCl on the absorptionproperties of Fe1Mo0.15(M).

4. Conclusion

CH4-CVD can be used to grow carbon nanotubes and nanofibers

on the surface of EV with FeMo impregnated catalysts. The

produced carbon/EV nanostructured composites showed high

efficiency to absorb different oils (soy bean, engine, diesel) with a

remarkable decrease on H2O absorption. These results were

discussed in terms of: (i) a strong hydrophobic character produced

by the carbon structures, (ii) an increase on surface area which

favours interaction with the oils and (iii) a sponge effect

produced by the entangled nanofibers and nanotubes favoring

capillarity. Moreover, the CVD process does not cause any

Fig. 4. XRD for the pure EV, EV/Fe2Mo0.30(M) after reduction and EV/Fe2Mo0.30(M)

after CVD process.

Fig. 5. Absorption of water and spilled oils (soy bean cooking, mineral oil and

diesel) by the hydrophobic vermiculitenanostructured carbon composites

and two other modified EV absorbents described in the literature [13] and

commercially available [17].

F.C.C. Moura, R.M. Lago/ Applied Catalysis B: Environmental 90 (2009) 436440 439


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significant change in the EV pore volume available for liquid

absorption. This novel floatable absorbent/adsorbent can be pro-

duced by a simple process using low cost and available chemicals

and showed very promising results to be used in environmental

remediation especially in oil spilled on waters.

Acknowledgements

The authors are grateful to CAPES, CNPq and FAPEMIG for

financial support and R. Martel for the laboratory facilities.

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catalytic growth of cnt and nano fibers on vermiculite

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